Electrophotographic image forming apparatuses, such as laser beam printers and digital copying machines, are provided with an optical scanning device that exposes a photosensitive member to light. The optical scanning device deflects a light beam emitted from a semiconductor laser by using a rotating polygon mirror that rotates and scans the light beam over the photosensitive member. An electrostatic latent image is thereby produced on the photosensitive member. Toner is attached to the electrostatic latent image for developing, so that a toner image is formed. The toner image is transferred to a sheet to form an image.
In recent years, the colorization of image forming apparatuses has been developed. For color image forming apparatuses, the so-called tandem-type, which includes photosensitive members that are each used for one color and collectively forms images in the respective colors on an intermediate transfer member, has been the mainstream type. In tandem-type color image forming apparatuses, the so-called four-in-one optical scanning devices, in which one rotating polygon mirror performs exposure for four colors, are widely used because of an advantageous unit size and cost.
Recent trends in image forming apparatuses that can be mentioned herein are an increase in speed and an increase in resolution, in addition to the colorization described above.
One measure to achieve the increase in speed and the increase in resolution is to rotate the rotating polygon mirror at a high speed. However, the rotation of the rotating polygon mirror at a high speed creates a high negative pressure near the rotating polygon mirror in the interior of the optical scanning device, so that it is easy for the optical scanning device to draw air from the exterior. The air drawn from the exterior of the optical scanning device may include fine dust and volatile matter in grease used in the image forming apparatus itself. When such air enters the optical scanning device, extraneous matter on the reflective surfaces of the rotating polygon mirror increases, and, within a period from several weeks to several months, an image failure may occur, for example, such that density is extremely decreased at part of the image due to a decrease in the amount of exposure light.
To prevent this, an elastic seal member made of synthetic rubber or polyurethane is attached to a cover for an optical housing at a junction that comes in contact with the outer periphery of the optical housing. The seal member is sandwiched between the cover and the optical housing to ensure the sealability of the interior of the optical housing. The seal member, as an independent component, is attached to the cover or the optical housing with a double-sided tape. For reliable sealing, however, it is necessary to carefully attach the seal member such that the seal member follows the shape of the cover or the optical housing. This operation is complicated.
In view of this, for example, PTL 1 discloses that an optical scanning device is configured such that a separated seal member is not attached, but an elastomeric seal member is injection-molded integrally on the optical housing or the cover so that an assembling process is simplified and the optical scanning device achieves sealability.